The present invention is directed to tantalum-based metal alloys, and more particularly to wire wrought from such alloys.
Electrolytic capacitors and furnace components in high temperature vacuum furnaces are major application areas for tantalum. Properties of tantalum that make it an attractive material in these applications include: high melting point, high dielectric constant in the tantalum oxide film formed by anodizing, good electrical conductivity, excellent fabricability and ductility, and availability in high purity forms. Other desirable properties or characteristics or tantalum for these applications are: resistance to embrittlement, fine grain size and resistance to grain growth, and good weldability, including joints to dissimilar materials.
Tantalum is known to become embrittled when exposed to gases such as oxygen, carbon monoxide and carbon dioxide for only short times at temperatures of about 315.degree. C. (600.degree. F.) and higher. These and other contaminating gases comprise the products or reactants of many physical and chemical reactions involving the use of tantalum products, either directly or indirectly, in the electronics, metal and chemical industries. The "embrittlement" condition refers to the loss of desired ability to bend without breaking in the intended application (as observed and measured at or near room temperature) resulting from exposure of tantalum to high temperatures in unsuitable vacuums or in contaminating gases and vapors. The lack of low temperature ability to bend without breaking after contamination causes severe problems when fabricated parts of tantalum that have been contaminated are subsequently exposed to vibration, impact and static forces at or near room temperature during use or subsequent manufacture.
One of the major difficulties in the use of tantalum in electrolytic capacitors has been that tantalum lead wires often become severely embrittled during sintering of slug-type tantalum anodes produced by pressing and sintering of tantalum powder with the tantalum lead wire embedded in the powder slugs. The extend of embrittlement is known to be more severe when such tantalum lead wires are embedded in tantalum powders having a relatively high oxygen content--for example, more than 1600 parts per million--and with powders sintered at temperatures of 1800.degree. C. or higher. Embrittlement of tantalum lead wires is a major problem when handling anodes that are welded to the rack employed in the anodizing process. Embrittlement is most severe in the area where the tantalum lead wire is embedded into the tantalum powder slug. Maximum or peak embrittlement is noted at the point of egress of the wire from the sintered anode, where the oxygen content in the wire is high and where the wire is unsupported. Embrittlement of tantalum lead wires is a major consideration in ability to withstand further capacitor manufacturing operations and handling. A solution to the tantalum lead wire embrittlement problem has a strong bearing on the ability to manufacture capacitors economically.
Furthermore, embrittlement of wrought tantalum fabricated components in high temperature furnace or other high temperature applications can adversely affect life of the parts. Tantalum materials in high temperature applications are adversely affected because they act as "getters" for contaminant gases such as carbon monoxide, carbon dioxide, oxygen and nitrogen. Grain growth at elevated temperature is also a significant problem. Coarse grain size tends to increase embrittlement and cracking when contaminated with relatively small amounts of oxygen. Replacement of tantalum parts because of embrittlement and failure can cause lengthy down-time and result in a sizable replacement costs. Substantial economical benefits can be gained if the service life of such tantalum parts can be increased.
One method that has been used to overcome this difficulty has been to treat the surface of the tantalum lead wire with carbon or a carbonaceous material. The carbon coating tends to react with oxygen in the tantalum powder during the subsequent high-temperature sintering operation, so that bendability of the lead wire is maintained because the oxygen has reacted with the carbon coating rather than being absorbed into the tantalum lead wire. However, it is difficult to control the application of carbon to obtain consistent properties and to maintain the desired bendability in the lead wire. In addition, carbon on the surface of the wire exerts an adverse effect on the electrical properties of the tantalum by producing an undesired increase in DC leakage through the tantalum oxide dielectric film of the resulting capacitor.
Still another method that has been used in an effort to lessen the extend of embrittlement of a tantalum lead wire is to use a grain-size-controlled tantalum lead wire--i.e., a tantalum wire that exhibits a grain size that does not grow significantly upon exposure to the elevated temperatures employed during sintering of the anode. However, a grain-size-controlled lead wire still does not possess the desired resistance to embrittlement in many instances, especially in those applications where the grain-size-controlled tantalum lead wire is embedded in a high-oxygen containing tantalum powder slug, and most especially where the oxygen content of the tantalum powder is 1600 ppm or higher.
In accordance with Marsh et al U.S. Pat. Nos. 4,062,679, 4,128,421 and 4,235,629, assigned to the assignee of the present invention and incorporated herein by reference, addition of sufficient silicon to provide from about 50 to 700 parts per million (ppm) silicon relatively uniformly distributed in the metal reduces the embrittlement of tantalum and tantalum-base alloys. The silicon-containing tantalum compositions are made by powder-metallurgy pressing and sintering techniques by first blending silicon into a master alloy blend of relatively high silicon content, and then blending the master alloy blend into the total composition. The final blend is pressed and sintered to produce a dense bar which is then fabricated as desired. In the fabrication of tantalum wire, the bar is subjected to multiple cold rolling steps, and then to multiple wire drawing steps, until wire of the desired diameter is obtained.
Torti U.S. Pat. No. 3,268,328, utilizes 10 to 1000 ppm addition of elements having atomic numbers 39 (yttrium) and 57 through 71 (lanthanum and rare earths) to provide ductile wrought tantalum and tantalum-alloy products with fine grain size which is resistant to grain coarsening at elevated temperatures. Douglass et al U.S. Pat. No. 3,497,402 discloses a process for producing a cold worked annealed tantalum alloy containing between about 10 and 1000 ppm yttrium which has a grain size finer than ASTM No. 3 upon heating to 2038.degree. C. (3700.degree. F.) for one hour. Good ductility and strength properties are also claimed for the yttrium-containing materials.
Other additives have been made to wrought metal products to achieve a fine initial grain size, an increased recrystallization temperature, and resistance to grain growth at elevated temperatures. Thoria and zirconia, which are very refractory oxides, remain in tungsten products through high-temperature sintering processes and restrain grain growth during heating at the operating temperature of a lamp filament. (Yih, S. W. H. and C. T. Wang, "Tungsten Sources, Metallurgy, Properties and Applications", Plenum Press, N.Y., 1979). Thoria and zirconia are added as a nitrate or chloride to the tungsten oxide, whether yellow, blue or brown, and depending upon the reduction process employed. The amount of thoria or zirconia may be up to 4 or 5% of the calculated final weight of tungsten metal powder to be reduced. After blending, the tungsten oxide is air dried and reduced in a hydrogen furnace. The reduced produce is screened and blended with pure tungsten powder as required in order to obtain a powder with 1 or 2% of the refractory oxide.
Good weldability is required in tantalum and tantalum alloy materials for many applications. In the case of tantalum lead wires for tantalum electrolytic capacitors, good weldability may be required in seal-welding the wire to the capacitor case, or joining the wire to another tantalum wire or to a metal wire such as nickel.